The present invention relates to a sewing machine which performs program control of sewing patterns, and more particularly to a movement control apparatus of a work holder of such sewing machines.
In some known sewing machines, sewing patterns are stored as coordinate values in memory medium, e.g., magnetic tape or magnetic card, and a work holder is moved in accordance with the sewing patterns information. In such sewing machines, a workpiece is moved to the next sewing point determined by the sewing patterns information while the needle is disengaged from the workpiece.
FIG. 1 illustrates a conventional movement control. So-called needle bar loci 1 and 2 are shown, in which needle bar locus 2 is twice the speed of the needle bar locus 1. Points A.sub.1 and A.sub.2 represent needle disengagement points and points B.sub.1 and B.sub.2 represent needle insertion points. Points C.sub.1 and C.sub.2 represent upper dead points which are highest points of the needle. In this stage, the movement period to move the work holder is determined between the needle disengagement point A.sub.1 or A.sub.2 and the needle insertion point B.sub.1 or B.sub.2. In the needle bar locus 1 (hereinafter referred to as "locus 1"), the period 3 is the movement period. In the needle bar locus 2 (hereinafter to as "locus 2"), the period 5 is the movement period. To move the workpiece within the movement period, necessary pulse number based on the coordinate values information of the sewing patterns is moved to X coordinate pulse motor and Y coordinate pulse motor and the work holder is moved to predetermined distance.
The pulse motors have an inherent delay time .alpha., so that when motor drive pulse is applied, the motor starts after the delay time .alpha.. Thus, in conventional movement control, movement pulse input timing is controlled previously by considering the delay time .alpha. of the pulse motor.
More particularly, in the known movement control, when the workpiece is to be sewn according to the sewing machine speed shown in the locus 1, a movement timing is set at a point 7 (hereinafter referred to as "movement point 7") having an upper shaft angle before the delay time .alpha. rather than the movement period 3 in order to move the workpiece within the movement period 3. Thus, when the upper shaft angle is reached at the movement point 7, a pulse generator applies a predetermined number of drive pulse train 8 synchronous with the sewing machine speed to the pulse motor. The pulse motor drives the work holder after the delay time .alpha. from receiving the drive pulse. Thus, drive period 9 of the work holder is within the movement period 3 as shown in FIG. 1, and the workpiece is moved accurately within the movement period 3.
In such known movement control, when the sewing machine speed is increased as shown in locus 2, a predetermined number of drive pulse train 8' is applied to the pulse motor synchronous with the sewing machine speed. The pulse initiates at the same movement point 7' of the upper shaft angle. As the upper shaft rotates twice in this case, time lapse from the point 7' to the needle disengagement point A.sub.2 is not the delay time .alpha.. That means, the work holder is not started to move at the needle disengagement point A.sub.2, and the pulse motor drive period 9' does not coincide with the movement period 5. Consequently, as shown in FIG. 1, needle insert time B.sub.2 may be within the drive period 9', so that stitch errror or needle breakage may result.